1. Field of the Invention
The present invention relates to a three-dimensional image display device which can display a three-dimensional image.
2. Related Art
Three-dimensional (3-D) image display techniques are classified to various types. When a 3-D image is displayed without using glasses by multiview system, holography, or integral photography system (hereinafter, called “IP system”), for example, the following constitution may be employed. That is, a plurality of pixels for a two-dimensional image display arranged two-dimensionally constitute each of pixels for a 3-D image display, and an optical plate is arranged on a front face of the pixels for a 3-D image display. In the optical plate, an exit pupil designed to be capable of taking only image information or data included in one pixel for a two-dimensional image display from the pixel for a three-dimensional image display is provided for each of pixels for a three-dimensional image display. That is, a viewer can view a three-dimensional image as an autostereoscopic view without using glasses by partially shielding pixels for a three-dimensional image display with the optical plate and making pixels for a two-dimensional image display observed by the viewer through exit pupils different at each viewing position.
In a detailed explanation about the IP system, an image displayed on a pixel for a three-dimensional image display is called “an elemental image”. The elemental image corresponds to a pin-hole camera image shot through a pin hole replacing the exit pupil.
Incidentally, an electronic device is lower in resolution than a silver film for a pin hole camera in the existing circumstances, and the term “elemental image” used in this text simply expresses a collection of pixels constituting plural two-dimensional images with different shot angles. That is, of elemental images displayed on individual pixels for a three-dimensional image display with the above constitution, namely, of a collection of constituent pixels for two-dimensional images (parallax images) shot in a plurality of different directions, only image information pieces coincident with an observing direction of a viewer, namely, only image information pieces which should be viewed when a three-dimensional image is actually present are viewed.
A difference between the multiview and the IP system occurs due to a low resolution of an electronic device. Though shooting angles for elemental images should be continuous ideally, they become discrete necessarily due to lack in resolution of an electronic device. In this case, the multiview system is designed such that lines connecting exit pupils and pixels, namely, light rays emitted via exit pupils are converged at a viewing distance, on the other hand, the IP system is not provided with a converging point of light rays.
In a binocular system is a three-dimensional image displaying system which adopts such a design that two-dimensional images acquired at respective shooting positions in a perspective projection manner are converged at a pair of points spaced from each other by an inter-eye distance (about 65 mm). According to this design, a viewer can view individual images (respective two-dimensional images shot at two shooting positions) with his/her right eye and left eye without using glasses at a position where he/she is spaced from a screen by an observation viewing distance L. Further, in a state that at least two pairs of converging points of light rays are arranged in side by side, an image observed with the left eye of a viewer and an image observed with his/her right eye are switched according to movement of a viewing position toward a left and/or a right direction. Accordingly, the viewer can confirm an aspect that a three-dimensional image varies according to movement of the viewing position.
On the other hand, the IP system is a three-dimensional image display system adopting such a design that two-dimensional images shot at respective shooting positions are not converged at one point. For example, such a design is employed that, assuming that a viewing position is spaced from a display plane by infinity, images observed with one eye of the viewer is switched to respective images shot at a plurality of shooting positions according to change in viewing angle. In a representative example, an image for three-dimensional image display can be produced using an image shot according to an parallel projection method by employing such a design that light rays emitted from different exit pupils become parallel.
According to such a design, since observation is not performed physically from a position spaced from a display plane by infinity, a two-dimensional image viewed with a viewer's one eye does not become equal to a two-dimensional image shot at any shooting position. However, a two-dimensional image observed with a viewer's right eye and a two-dimensional image viewed with his/her left eye are each constituted by summing images shot from a plurality of directions by an parallel projection method, so that they averagely become two-dimensional images shot from a viewing position by a perspective projection method. With such a constitution, individual images can be viewed with the right eye and the left eye, and a three-dimensional image which a viewer perceives becomes equivalent to a three-dimensional image recognized when the viewer actually observes a shot object from either direction. That is, the viewing position is not assumed in the IP system.
Regarding the IP system, the multiview system, and further a dense multiview system where converging points are provided at a pitch shorter than an inter-eye distance, differences among them in occurrence of display blocking generated due to a non-display portion in a two-dimensional image display device will be explained.
According to the above-described constitution, in the IP system, a position of a pixel appearing via an exit pupil when observation is performed from one point at a viewing distance is unstable. That is, a position where a pixel is observed varies periodically for each exit pupil. In the IP system, therefore, exit pupils which can view a non-display portion (a boundary between pixels) distribute within a screen periodically and they are viewed as a luminance change within the screen. On the other hand, since the multiview system is designed such that light rays converge at a viewing distance, the same position of a pixel can be viewed from all exit pupils when viewing is performed at one point at the viewing distance. Since the number of pixels converging rays is reduced to 1/n, even when the same position in a pixel is observed from exit pupils of 1/n all exit pupils, even in the dense multiview system where the number of converging points of light rays has been increased to n times thereof. That is, in the IP system, a non-display portion can not be viewed from each exit pupil at all. Alternatively, such a state where only the non-display portion can be viewed from all the exit pupils occurs. In other words, a luminance unevenness or moire occurring due to a non-display portion within a screen in the IP system occurs as luminance distribution unevenness in a space where a viewer is present in the multiview system. In the dense multiview system where a non-display potion can always be viewed at a constant ratio to movement of a viewing position in a horizontal direction by reducing the number of exit pupils which can view a pixel center at a viewing distance to 1/n and increasing the number of the converging points of light rays to n times, luminance change in a space at the viewing distance can be suppressed, but unless an aperture ratio of a two-dimensional image display device or an aperture ratio of an exit pupil (a window portion) in an optical plate is controlled, the luminance change can not be removed completely (for example, refer to U.S. Pat. No. 6,064,424).
In each system of the IP system, the multiview system, and the dense multiview system, since a plurality of pixels for a two-dimensional image display is used to constitute a pixel for a three-dimensional image display, a resolution of a three-dimensional image to be reproduced becomes lower than an original resolution of an electronic device. For purpose of suppressing such a resolution reduction, a method which provides only parallax information or data pieces in a horizontal direction which is effective for acquiring a stereoscopic feeling has been already studied (a one-dimensional IP system and a one-dimensional multiview system). In this case, exit pupils in an optical plate are formed so as to be continuous in a vertical direction, and elemental pixels are developed only in a horizontal direction. That is, the parallax information pieces are not provided in the vertical direction. Regarding the resolution reduction in the horizontal direction, such a method has been studied focusing attention on a pixel arrangement in an electronic device such as an LCD that a resolution in a vertical direction is decreased to ⅓ but a resolution in a horizontal direction is increased to three times by handling a triplet of RGB adjacent to one another vertically as one pixel instead of conventionally handling a triplet of RGB adjacent to one another horizontally as one pixel (for example, refer to JP-A-10-253926).
As described above, in addition to the problem about the resolution lowering, there is the problem about the display blocking due to a non-display portion of a two-dimensional image display device generated by an action between the non-display portion and an optical plate. Regarding this problem, U.S. Pat. No. 6,064,424 has proposed that an optical plate is inclined for solving the display blocking and improving a resolution balance in a three-dimensional image display device of a multiview system. By inclining the optical plate, a region where a non-display portion can be observed at the viewing distance is dispersed spatially, so that a luminance change can be suppressed. However, since the non-display portion in the electronic device is not formed in a vertical stripe shape but in a grating shape, it is necessary to incline a lens and control a numerical aperture of an exit pupil (a window portion) in an optical plate in a two-dimensional image display device strictly in order to suppress the luminance change due to the non-display portion completely. Further, since the vertical resolution can be distributed to the horizontal resolution to a certain extent by inclining the optical plate, a method for improving deterioration in resolution balance by providing parallax information pieces only in the horizontal direction has been described in U.S. Pat. No. 6,064,424.
However, it can not be perceived from the following reason that the resolution balance has been improved sufficiently. When a three-dimensional image is displayed by distributing a plurality of parallax images which are viewable regions in such a system as the IP system or the multiview system, such a problem arises that a resolution in the three-dimensional image display device lowers from an original resolution in an employed two-dimensional image display device due to a constitution of the system and the display blocking occurs due to the non-display portion.